1. Field of the Invention
The present invention relates to a control device for a hybrid vehicle which jointly uses an engine and two electric motors. More particularly, the invention pertains to a control device for a hybrid vehicle having the ability to switch it between series drive mode and parallel drive mode depending on running conditions.
2. Description of the Related Art
Among vehicles including motor vehicles, hybrid vehicles which jointly use an engine and electric motors have been developed in recent years due to their low-pollution and resource-saving features. These hybrid vehicles employ many techniques for achieving an improvement in kinetic energy recovery efficiency and for maintaining running performance by installing two motors, one for power generation and the other as a power source.
One known example of a hybrid vehicle of this kind is made such that it is switched between series drive operations, in which the hybrid vehicle runs while driving a motor for electric power generation with mechanical output of an engine and then driving a motor for running with both generated electric power and electric power output discharged from a battery, and parallel drive operation, in which the hybrid vehicle runs mainly with the mechanical output of the engine and the difference between required output and the mechanical output of the engine is made up for by means of the motors, depending on running conditions.
As an example, Japanese Patent Laid-Open No. 98322/1996 discloses a series-parallel hybrid electric vehicle comprising an engine, a generator driven by mechanical output of the engine, a battery charged with electricity generated by the generator, a motor driven by electric power discharged from the battery, and coupling/decoupling means like a clutch for establishing and breaking a mechanical link between the generator and the motor.
In the aforementioned series-parallel hybrid electric vehicle, series drive operation is selected when the clutch is engaged, while parallel drive operation is selected when the clutch is disengaged, and the rotating speed of the generator is matched with that of the motor before the clutch is engaged to avoid shocks caused by its engagement.
It is, however, likely that fluctuation in toque occurs due to a difference in output torque between the engine and motor sides at the time of switching from series drive operations to parallel drive operation if the clutch is engaged simply upon matching the rotating speed of the generator with that of the motor. Fluctuation in toque may also occur at the time of switching from the parallel drive operation to the series drive operation if the clutch is simply disengaged.